Curable silicone compositions are used as sealing materials or protective coating materials for optical semiconductor elements in optical semiconductor devices such as light emitting diodes (LEDs). However, since the gas permeability of a cured product of a curable silicone composition is high, problems such as the discoloration of the sealing material due to corrosive gas and the reduction of brightness due to the corrosion of silver plate on the LED substrate occur when used in a high-brightness LED with high optical intensity and a large amount of heat generation.
Therefore, a curable silicone composition comprising a branched-chain organopolysiloxane having methylphenylvinylsiloxane units, an organohydrogenpolysiloxane, and an addition reaction catalyst is proposed in Japanese Unexamined Patent Application Publication No. 2012-052045 as a curable silicone composition which forms a cured product with low gas permeability.
However, such a curable silicone composition yields a slow curing reaction, and when curing is insufficient, tackiness occurs on the surface of the cured product, which is problematic in that dirt or dust adheres to the surface. In addition, there is also the problem that a large amount of time and energy are required to complete the curing reaction of such a curable silicone composition.
An object of the present invention is to provide an organopolysiloxane having high hydrosilylation reactivity and forming a cured product with low gas permeability, and further to provide a curable silicone composition having high reactivity and forming a cured product with low gas permeability. Another object of the present invention is to provide a cured product with low gas permeability, and further to provide an optical semiconductor device with excellent reliability.